Permanent magnet translational motor with auxiliary electromagnet

ABSTRACT

A permanent magnet translational motor includes a double-ended cylindrical outer shell having a hollow interior, a solid cylindrical core in the hollow interior coaxially aligned with the longitudinal axis of the shell, and permanent magnet segments secured to the interior surface of the shell so as to surround the core. The permanent magnets develop a radial magnetic field across the air gap between the magnets and the core. A moving drive coil mounted on the core slides longitudinally back and forth in the magnetic field in response to changes in electric current passing through the coil. Opposed mounting brackets on the moving drive coil extend outwardly through longitudinally extending slotted openings in opposite sides of the shell for connection to a suitable load. In a preferred application, the motor provides reciprocating translational motion for a piston in a respirator. A stationary auxiliary electromagnet located in one end of the shell interior is connected to a source of electrical power independent of the electrical power source to which the movable drive coil is connected. The auxiliary electromagnet varies the magnetic field across the air gap in response to changes in electric current passing through it to alter (increase or decrease) the maximum force generated by the motor.

United States Patent 11 1 Gillott et a1.

[ 1 Jan. 28, 1975 1 1 PERMANENT MAGNET TRANSLATIONAL MOTOR WITH AUXILIARY ELECTROMAGNET [75] Inventors: Donald H. Gillott; James G. Simes,

both of Sacramento, Calif.

[73] Assignee: Sutter Hospitals Medical Research Foundation, Sacramento, Calif.

22 Filed: Jan. 15, 1973 211 Appl. No.: 323,501

[52] US. Cl 310/27, 417/417, 128/145.6 [51] Int. Cl. H02k 33/18 [58] Field of Search 310/12 l9,

[56] References Cited UNITED STATES PATENTS 2,750,961 6/1956 Uritis 310/27 X 2,931,925 4/1960 Diilz 310/27 3,161,793 12/1964 Laithwaite 310/27 3,439,198 4/1969 Lee 310/27 X 3,440,464 4/1969 T0lmie.... 310/27 3,505,544 4/1970 Helms 310/13 3,603,307 9/1971 Bischler 128/1456 3,659,124 4/1972 Lathrop 310/27 X 3,666,977 5/1972 Helms et a1 310/27 X 3,681,630 8/1972 Sutton 310/16 3,723,779 3/1973 Gillum 310/27 X 3,723,780 3/1973 Gillum 310/13 3,781,140 12/1973 Gladden 417/417 X 38 AUX/MR) :0/4 value:

Primary ExaminerDonovan F. Duggan Attorney, Agent, or Firm-Christie, Parker & Hale 1 1 ABSTRACT A permanent magnet translational motor includes a double-ended cylindrical outer shell having a hollow interior, a solid cylindrical core in the hollow interior coaxially aligned with the longitudinal axis ofthe shell, and permanent magnet segments secured to the interior surface of the shell so as to surround the core. The permanent magnets develop a radial magnetic field across the air gap between the magnets and the core. A moving drive coil mounted on the core slides longitudinally back and forth in the magnetic field in response to changes in electric current passing through the coil. Opposed mounting brackets on the moving drive coil extend outwardly through longitudinally extending slotted openings in opposite sides of the shell for connection to a suitable load. In a preferred application, the motor provides reciprocating translational motion for a piston in a respirator. A stationary auxiliary electromagnet located in one end of the shell interior is connected to a source of electrical power independent of the electrical power source to which the movable drive coil is connected. The auxiliary electromagnet varies the magnetic field across the air gap in response to changes in electric current passing through it to alter (increase or decrease) the maximum force generated by the motor.

23 Claims, 5 Drawing Figures PERMANENT MAGNET TRANSLATIONAL MOTOR WITH AUXILIARY ELECTROMAGNET BACKGROUND OF THE INVENTION This invention relates to an improved permanent magnet motor, and also to an improved air delivery system for respiratiors in which a permanent magnet motor provides the linear drive for the respirator piston.

In the past, permanent magnet motors generally have been used in servo systems to apply translational motion to objects whose linear position is to be controlled. A permanent magnet motor generally includes a moving coil mounted on a core of ferromagnetic material which, in turn, is placed in a magnetic field produced by a permanent magnet. Electric current passing through the coil interacts with the magnetic field of the permanent magnet to produce a translational force on the coil parallel to the longitudinal axis of the core to move the coil, anda load attached to the coil, along a linear path. Periodic reversal of the current in the coil moves the coil in a reciprocating fashion to apply reciprocating longitudinal movement to-the load.

One typical use of a permanent magnet translational motor is for driving a pen in a strip chart recorder. Such a motor is used in the strip chart recorder Models 71m and 7143B manufactured by Hewlett Packard relatively low intensity is produced across the gap. This develops a relatively small motor output force of about 1 pound. Then pen drive motor also has a stroke length of about five to ten inches. A oke of such n t s a t i dnbs ausgth magnetic field intensity and the current developed in the moving coil are both of such low magnitude that the coil can be moved axially through the 5 to inch stroke without the core being driven into saturation. Because the motor generates a small force, the moving coil may be coupled to the pen unsymmetrically.

Other permanent magnet translational motors have been designed for applications requiring a relatively large output force, say about 30 to 40 pounds. A typical motor used in such applications is the Model 14 computer memory head positioning motor manufactured by Information Magnetics Corporation. This motor includes a ferromagnetic housing with one open end, and a sliding coil mounted on a relatively short, cylindrical steel core inside the housing. The coil moves back and forth in a narrow air gap, which enables a relatively large flux to be developed across the gap. The current passing through the coil interacts with the large flux to produce an output force of about 40 pounds at a stroke length of about two inches. The motor is not suitable for use in applications requiring a long stroke length in the neighborhood of about five to ten inches, because the relatively large field over the longer length drives the core into saturation, for motors with stroke lengths of more than 2 inches, unless a larger cross-sectional area of iron is employed.

SUMMARY OF THE INVENTION This invention provides a permanent magnet motor which produces a relatively large output f rce say more than 40 pounds, through a relatively long stroke length in the neighborhood of about 5 to 10 inches. A typical use for such amotor is for driving the piston of a respirator, where the combination ofa relatively large force and long stroke length is necessary to force a given volume of air into the lungs of a patient under a predetermined pressure, volume, and/or flow. The rcspirator also includes electronic controls for adjusting the output force of the motor so the pressure. volume. and/or flow of the air supplied to each patient can be easily and accurately adjusted.

Briefly, the motor includes a shell with a pair of closed ends enclosing a hollow interior, and an elongated core extending through the hollow interior from one end of the shell to the other. A permanent magnet disposed in the interior of the shell surrounds a predetermined length of the core. The permanent magnet is spaced from the core to form a narrow air gap between the magnet and the core so that a magnetic field is established radially across the air gap. A movable coil surrounding the portion of the core located in the air gap is supported on the core to slide longitudinally back and forth relative to the core in the magnetic field in response to changes in electric current passing through the coil. The coil has opposed mounting means extending outwardly from it through corresponding openings in the shell for attachment to a load. Reciprocating translational motion of the coil through the field is transferred to the load.

In a preferred form of the invention, an auxiliary coil is disposed at one end of the shell. The magnetomotive force generated by the auxiliary coil may either aid or oppose the magnetomotive force of the moving coil, thereby altering the air gap flux for large values of movable coil currents. By proper choice of auxiliary coil current, the maximum available force may be varied without affecting the small current characteristics of the movable coil.

The motor develops a magnetic field of relatively high intensity because (1) the permanent magnet has a relatively high magnetic flux retentivity, (2) the air gap between the coil and permanent magnet is relatively narrow, so that the flux developed by the permanent magnet traverses the air gap but not the leakage path, and therefore is not readily dissipated, (3) the core is of relatively large size so that it can support the magnetic flux, and (4) the shell is closed at both ends, which reduces by one-half the magnetic flux which would exist in the magnetic parts of a motor with one end open. The coil has a relatively high currentcarrying capacity, and the relatively high current interacts with the high intensity magnetic field to generate a relatively large output force in excess of more than about 40 pounds.

The auxiliary coil varies the magnetic saturation in the core, and therefore overcomes the tendency of the core to become saturated when the moving coil carries large current as it moves through the magnetic field. This increases the force produced by the movable coil for a given stroke length. Thus, the auxiliary coil can be coupled to an electronic control circuit which, in turn, can be used to adjust the maximum force applied by the coil. This means of adjustment can be important in applications such as respirators, where the pressure, volume, and/or flow developed by the moving respirator piston should be adjustable to meet the needs of varie patisats-a The use of the permanent magnet motor in combination with a respirator provides several advantages over prior art respirators. For example, the piston in prior art respirators generally is driven by a rotary drive motor which requires gear reduction, together with relatively complicated mechanical linkages, such as a rack, pinion, joints, and the like, to convert the high frequency rotary motion of the motor into the relatively low frequency linear motion of the piston. On the other hand, the permanent magnet motor of this invention provides a respirator drive having no moving parts other than the moving coil assembly of the motor and the piston. This assures extreme mechanical simplicity, reliability and accurate control when compared with prior art respirators.

The permanent magnet translational motor also provides essentially noisefree operation, which is a substantial improvement over prior art respirators which operate at a constant noise level and therefore can be uncomfortable for the patient and the hospital attendants.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will be more fully understood by referring to the following detailed description and the accompanying drawings in which:

FIG. I is a fragmentary schematic elevation view, partly in cross-section, showing a permanent magnet motor in accordance with this invention for driving a piston in a respirator;

FIG. 2 is a fragmentary cross-sectional elevation view of the permanent magnet motor;

FIG. 3 is a cross-sectional plan elevation view taken on line 3-3 of FIG. 2;

FIG. 4 is a fragmentary elevation view showing a movable coil assembly for the permanent magnet motor; and

FIG. 5 is a graph showing the relationship between the force produced by the motor and the current in the moving coil of the motor as a function of the current in the motors auxiliary coil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIIMENT Referring to FIG. 1, an air delivery system for a respirator includes a piston 12 disposed in a cylinder 14. The piston is driven vertically back and forth in the cylinder by a permanent magnet motor 16 coupled to the piston by suitable means to be described in detail below.

A supply of gas, usually air, or a suitable mixture of air and oxygen, is delivered to the interior of cylinder 14 through a non-rebreathing valve 18 and port 20. When the permanent magnet motor pulls the piston downward in the direction of the arrows shown in FIG. 1, the gas is forced out through port 20 of cylinder 14, through non-rebreathing valve 18, and through a line (not shown) connected to a patient (not shown) in a conventional manner for delivering the gas to the patients lungs. The reciprocating movement of the piston periodically pumps the gas into the patient's lungs to replace the inspiratory cycle of normal breathing and allow suitably adjusted time for expiration. The expiratory cycle occurs each time the piston is retracted, with the patient exhaling passively through a separate conduit (not shown) as is well known in the art of respirator therapy.

As shown best in FIGS. 2 and 3. permanent magnet motor 16 includes an elongated open-ended cylindrical main shell 22 which is preferably about 14 inches long, with an inside diameter of about 5 inches and a wall thickness of about five-eighths inch.

An elongated solid cylindrical core 24 is disposed centrally within the bore of main shell 22. Core 24 is preferably about 14 inches long, with an outside diameter of about 3% inches. The core and main shell cooperate to form a narrow annular working air gap 26 between them having a width of about three-fourths inch.

The bottom ends of main shell 22 and core 24 are seated in cooperating annular and circular recesses 28 and 30, respectively. formed in the upper surface of a cylindrical bottom end plate 32. The bottom end plate has the same outside diameter as main shell 22 and is approximately 1% inches thick.

An open-ended cylindrical auxiliary shell 34 is fitted over the top of main shell 22. Auxiliary shell 34 is preferably about 3% inches in length, with an inside diameter identical to that of main shell 22. The top of shell 22 is seated in an annular recess 36 in the bottom end of shell 34. The wall thickness of the auxiliary shell is slightly greater than that of main shell 22 to accommodate recess 36.

An auxiliary electromagnet 38 is disposed within the bore of auxiliary shell 34. Electromagnet 38 includes an elongated solid cylindrical auxiliary core 40 having a slightly greater diameter than core 24 and a length almost identical to that of auxiliary shell 34. Auxiliary core 40 contains a circular recess 44 which seats over core 24.

A cylindrical top end plate 47 covers the top of electromagnet 38. Top end plate 47 is the same size as bottom end plate 32. The top ends of auxiliary shell 34 and auxiliary core 40 are seated in circular and annular recesses 48 and 50, respectively, formed in thebottom surface of end plate 47.

An auxiliary coil winding 51 of electromagnet 38 is wound from end to end on core 40 and occupies the approximately inch wide gap between the core and the interior of auxiliary shell 34. The bottom of winding 51 is supported by an annular plate 52 bonded to the bottom annular surface of the wound coil. Preferably, winding 51 comprises I075 turns of 18 gauge wire. A pair of leads 54 (see FIG. I) from winding 5I are connected to a power supply (not shown) and appropriate circuitry (not shown) to provide proper control described in detail below.

The assembled elements comprising the bottom portion of the permanent magnet motor are joined together by an elongated bolt 55 threaded into matching internally threaded bores 56 and 58 coaxially aligned in the bottom plate 32 and the bottom portion of core 24, respectively. The assembled elements comprising the top of the motor are joined together by an elongated bolt 59 threaded through a first pair of longitudinally aligned internally threaded bores 60 and 61 extending through end plate 47 and core 40, respectively.

The main structural components of the permanent magnet motor, namely main shell 22, core 24, bottom end plate 32, auxiliary shell 34, auxiliary core 40, and top end plate 47 are made of ferromagnetic materials, preferably steel with a high saturation flux density, to provide the magnetic circuit described in greater detail below.

A movable drive coil assembly 64, shown in crosssection in FIG. 2 and in elevation in FIGS. 3 and 4, is mounted on core 24 so as to be movable longitudinally relative to the core in the working air gap. The movable drive coil assembly includes a pair of relatively thin, elongated, tubular coil elements 65 mounted one on top of the other and disposed around core 24. Coil elements 65 comprise a continuous winding of aluminum ribbon coiled to form an elongated self-supporting ring. Both coil elements 65 together form a movable coil preferably about 6 inches in length from top to bottom. The coils preferably are the type of aluminum ribbon coil used in the Model 14 Head Postion System available from Information Magnetics Corporation. The coils are sized so their inside diameter is closely spaced from the outside diameter of core 24, the preferred spacing being about one-sixteenth-inch.

Coils 65 are attached to a carriage 66 comprising a rigid framework formed by a pair of vertically spaced apart, collinear top and bottom rings 68 and 70, respectively, and a pair of opposed vertically extending elongated carriage bars 72 secured to the opposite outer edges of the top and bottom rings. Preferably, the top and bottom rings, and their associated carriage bars, are made of aluminum. The top and bottom rings have the same diameter as coils 65, and are held in place tightly against the top and bottom of the coils by the carriage bars. The rings have a narrow longitudinal insulating strip (not shown) to prevent circulating current. The carriage rides vertically back and forth in air gap 26 when coil assembly 64 moves longitudinally relative to core 24.

The carriage travel is guided by a pair of opposed top ears 74 projecting inwardly from the inner peripheral edge of top rings 68, and-a pair of opposed bottom ears 76 projecting inwardly from the inner peripheral edge of bottom ring 70. Both sets of ears are fitted into corresponding longitudinally extending guide channels 78 formed in opposite sides of core 24. Each guide channel includes a U-shaped liner 80 fitted around a corresponding pair of the top and bottom guide ears. Each liner 80 is made of a low-friction material, preferably teflon.

Corresponding horizontally extending forcedistributing ears 82 project outwardly from carriage bars 72. The force-distributing ears extend through corresponding vertically extending elongated slotted openings 84 formed in opposite sides of main shell 22. The force-distributing ears carry corresponding vertically extending mounting pins 86 attached to respective elongated rods 88 (see FIG. 1) which, in turn, are connected to a piston rod 90 for driving piston 12 back and forth in cylinder 14 in response to longitudinally reciprocating travel of movable coil asssembly 64.

A pair of flexible lead wires 92 from the winding of coils 65 are connected to a power supply (not shown) and appropriate circuitry (not shown). Lead wires 92 provide sufficient slack to accommodate movement of coil assembly 64 within air gap 26.

Several sets of arcuately curved permanent magnet segments 98 are secured to the inner wall surface of main shell 22 to form the equivalent of an approximately 12-inch long, cylindrically curved, annular permanent magnet surrounding core 24. Preferably, four columns of permanent magnet segments are mounted inside shell 22, each column containing four permanent magnet segments, each segment being approximately three inches long. The permanent magnet segments are secured to each other and to the inner wall of the shell by a suitable bonding material such as epoxy resin. The permanent magnet segments form a continuous annular permanent magnet surrounding the core. except that a pair of narrow, vertically extending spaces 100 are formed between the segments along opposite sides of the shell in alignment with slotted openings 84. The two spaces accommodate the carriage bars 72 which ride in respective slotted openings 84 as coil assembly 64 moves relative to core 24. Each permanent magnet segment preferably is about five-eighths inch in thickness, which provides an air gap of about threesixteenths inch between the inner surface ofthe permanent magnets and the outer surface of core 24 for accommodating coil elements65 of the movable coil assembly.

The annular permanent magnet develops a radial magnetic field across working air gap 26. When the lead windings of the movable coil are connected to a suitable power supply, a tangential current flows through the moving coil. This current interacts with the radial magnetic field to produce a translational force on the movable coil assembly parallel to the longitudinal axis of core 24. The force developed by the movable coil is uniformly brought out of the motor by the opposed force-distributing ears 82 which transmit the force, through rods 88, to the respirator piston.

The respirator of this invention is capable of delivering 2 liters of air at a maximum pressure of 100 cm. H O. The relatively long stroke (about 10 inches) of motor IO-insures delivery of the desired two liter volume of air. A motor output force of about 42 pounds will produce the desired 100 cm H 0 maximum pressure. Generally speaking, an output force up to about 36 pounds will produce the pressures necessary for most patients needing respirator therapy.

The flux density of the permanent magnet segments, together with the size of core 24 and the size of air gap 26, are critical in enabling the motor to produce the relatively long stroke and an output force up to approximately 36 pounds without use of the auxiliary winding. The permanent magnet segments are made of a ferrite M-7 material manufactured by Allen Bradley Co., with a flux density of 3,400 gauss and a coercive force of 3250 oersteds. Steel core 24 has a cross-sectional area of approximately 9 square inches and a length of about 14 inches, which provide a sufficient cross-sectional area of ferromagnetic material to support the flux required to generate the relatively large force necessary for the respirator without driving the core into saturation. The relatively high flux density of the annular permanent magnet, together with the relatively narrow (3/16 inch) air gap between the permanent magnet segments and core 24, combine to produce the large magnetic flux required for the respirator drive. (The narrow air gap prevents most of the flux generated by the permanent magnets from being dissipated.) The relatively narrow wall thickness of the aluminum coil elements 65 allows such a narrow air gap to be used. (If the movable coil is a continuous winding of conventional insulated conductor wire, the air gap would have to be widened to accommodate the coil, and a wider air gap would reduce the effective flux available to obtain the large output force.)

The flux path of the main permanent magnet field is radially across air gap 26, symmetrically in both directions through core 24 toward the ends of the core, radially across both end plates 32, 47, and in the walls of main shell 22 and auxiliary shell 34. The combined wall thickness of the main outer shell, auxiliary shell, and end plates provides an area of ferromagnetic material at least equal to the area of core 24, and therefore is sufficient to support the flux carried in core 24.

The two closed ends of the motor shell concentrate the magnetic lines of flux to the area within the shell, which enables the relatively long stroke length to be obtained. When compared with a motor having one closed end, the present motor reduces substantially the number of permanent magnets required, and the volume of ferromagnetic material necessary to produce the desired force and stroke length.

Auxiliary electromagnet 38 varies the maximum output force developed by the motor. Electric current flowing through the auxiliary coil winding provides additional magnetomotive force which controls the magnetic saturation in auxiliary core 40 and top of core 24. The direction and magnitude of the current flow in the auxiliary winding can be adjusted to effectively add flux to take core 40 out of saturation. This additional flux accommodates greater current flow in the moving coil, and in effect adds to the output force developed by the motor.

The graph shown in FIG. illustrates how the current in the auxiliary winding can vary the output force of the motor over a relatively wide range. The motor develops an output force of 36 pounds when there is no current flow in the auxiliary winding. As discussed above, this force allows the respirator to provide the desired 2 liter volume of gas at pressures necessary for most patients needing respirator therapy. The graph of FIG. 5 shows that the current in the auxiliary coil can be increased to the point where the motor produces a maximum force of about 46 pounds. This output force provides 2 liters of gas at a pressure of more than 100 cm. H O, which easily meets the requirements of patients under respirator therapy.

The permanent magnet motor by its nature provides a simple means for controlling movement of the piston to deliver a controlled, time-related volume of gas to the patient throughout each inspiratory cycle. The purpose of the desired time-dependent adjustment of the piston movement is to provide the capability of controlling the pressure build-up in the patients lungs during each inspiratory cycle to prevent excessive pressure build-up which, in turn, could cause injury to the patients lungs. The auxiliary electromagnet allows the maximum force available from the motor to be electronically controlled. That is, the instantaneous pressure build-up in the patients lungs during each inspiratory cycle can be monitored, and the desired timedependent pressure within the patients lungs can be controlled by external circuitry (not shown) which alters the current flow through the auxiliary coil. This provides an immediate adjustment in the force developed by the respirator, and the force, in turn, controls the instantaneous pressure build-up in the patients lungs.

The use of the auxiliary coil also produces a larger output force than would be possible if additional permanent magnets alone were used to gain the additional output force. Additional permanent magnets alone would not produce an increase in output force unless the cross-sectional areas of core 24 and shell 22 were increased sufficiently to accommodate the additional flux. The auxiliary coil provides a cost savings in material when compared with the cost of additional permanent magnets and material for the core and outer shell.

The permanent magnet motor described herein also is especially advantageous for use as the drive for a respirator piston. The respirator provided by this invention has no moving parts, other than the piston itself and the movable coil assembly. No racks, joints, pinions, gear reduction, or other similar movable parts used in conventional respirators are necessary. The result is a respirator of extreme mechanical simplicity, reliability and accurate control. Moreover. this motor is essentially silent during operation, which is a substantial improvement over the noisy rotary type motors commonly used in respirators.

We claim:

1. A permanent magnet translational motor comprising a shell having an outer wall with a pair of closed ends enclosing a hollow interior, an elongated core extending through the hollow interior of the shell from one closed end to the other, permanent magnet means in the interior of the shell surrounding a predetermined length of the core, the permanent magnet means being spaced laterally from the core to form a narrow working air gap between the permanent magnet means and the core, with a magnetic field being established across the working air gap, the core having a first portion of its length located in the-working air gap and a second portion of its length located adjacent to but outside of the working air gap, a movable drive coil located in the working air gap and surrounding the first portion of the core, the drive coil being connected to a first source of electrical power and being movable back and forth relative to the first portion of the core in the magnetic field in response to changes in electric current passing through it, the drive coil having opposed mounting means extending outwardly therefrom through corresponding openings in the shell outer wall for attachment to a load for applying force to the load in response to movement of the drive coil, and an auxiliary coil located within the interior of the shell and wound around the second portion of the core such that the drive coil is movable toward and away from the auxiliary coil, the auxiliary coil being connected to a second source of electrical power independent of the power source associated with the drive coil for varying the magnetic field across the working air gap in response to changes in electric current passing through the auxiliary coil to increase the maximum force which may be developed by the drive coil.

2. Apparatus according to claim 1 in which the magnetic field established by the permanent magnet means defines a closed path extending in opposite directions along the core, across the closed ends of the shell, and along the outer wall of the shell.

3. Apparatus according to claim 1 in which the shell is cylindrical, and the core is a cylindrical body extending coaxially along the longitudinal axis of the cylindrical shell, the drive coil being an annular body adapted to move back and forth in an enclosed annular working air gap formed between the permanent magnet means and the core.

4. Apparatus according to claim I in which the auxiliary coil is independent of the drive coil and held in a stationary position in the shell interior.

5. Apparatus according to claim 1 in which the core is a solid steel body having an outer surface, the working air gap is a void which surrounds the core and extends from the outer surface of the core to the permanent magnet means, and the width of the working air gap is just sufficient to allow passage of the drive coil between the outer surface of the core and the permanent magnet means.

6. Apparatus according to claim 5 including a cooperating shoulder and groove arrangement between the drive coil and core for producing guided movement of the drive coil along the core.

7. A permanent magnet translational motor comprismg a closed steel shell having an outer wall with a pair of closed ends enclosing a hollow interior,

a core comprising an elongated solid steel body extending continuously through the hollow interior of the shell from one closed end to the other, the core having an outer surface spaced from the interior of the shell outer wall,

permanent magnet means located in the interior of the shell and surrounding a predetermined length of the core, the permanent magnet means being spaced from the core to form a narrow working air gap comprising a void which surrounds the core and extends from the outer surface of the core to the permanent magnet means, whereby a magnetic field is generated across the narrow working air gap, the core having a first portion of its length disposed in the working air gap and the second portion of its length adjacent to but outside of the working air gap,

a movable drive coil located in the working air gap and surrounding the first portion of the core, the width of the working air gap being just sufficient to allow passage of the drive coil between the outer surface of the core and the permanent magnet means, the drive coil being connected to a first source of electrical power so it will move back and forth relative to the first portion of the core in the magnetic field in response to changes in electric current passing through it,

opposed drive means extending outwardly from the drive coil through corresponding openings in the shell outer wall for attachment to a load to supply force to the load in response to movement of the drive coil, and

a stationary auxiliary coil located in the interior of the shell and wound around the second portion of the core, the auxiliary coil being adjacent to but outside of the working air gap so that the drive coil is movable toward and away from the stationary auxiliary coil, the auxiliary coil being independent of the drive coil and being connected to a second source of electrical power independent of the power source associated with the drive coil for varying the magnetic field across the working air gap in response to changes in electric current passing through the auxiliary coil to increase the maximum force which may be developed by the drive coil.

8. Apparatus according to claim 7 in which the magnetic field established by the permanent magnet means defines a closed path extending in opposite directions along the core, across the closed ends of the shell, and along the outer wall of the shell.

9. Apparatus according to claim 7 in which the drive means extend radially outwardly through corresponding longitudinally extending slotted openings in the outer wall of the shell; and including a cooperating shoulder and groove arrangement between the core and the drive coil for producing guided movement of the drive coil along the core.

10. An air delivery system for a respirator which includes a closed cylinder having a hollow interior, a piston which in normal use is movable vertically up and down in the interior of the cylinder for forcing air to a patient through an air outlet in the cylinder, and a linear drive for moving the piston, the linear drive comprising a closed steel shell adjacent the cylinder, the shell having an outer wall with a pair of closed ends enclosing a hollow interior of the shell, an elongated core extending vertically through the hollow interior of the shell from one closed end to the other, the core being a solid steel body having an outer surface spaced from the interior of the shell outer wall,-permanent magnet means in the hollow interior surrounding a predetermined length of the core, the permanent magnet means being spaced from the core to form a narrow working air gap comprising a void which surrounds the core and extends from the outer surface of the core to the permanent magnet means, whereby a magnetic field is established across the narrow'working air gap, a movable drive coil surrounding the portion of the core located in the working air gap, the width of the working air gap being just sufficient to allow passage of the drive coil between the outer surface of the core and the permanent magnet means, guide means for slidably engaging the drive coil with the core to allow the coil to move up and down vertically on the core and in the magnetic field in response to changes in electric current passing through the drive coil, drive means extending outwardly from the drive coil through corresponding elongated openings in the shell outer wall and connecting the piston to the drive coil to apply reciprocating translational motion to the piston inresponse to vertical movement of the drive coil, and an auxiliary coil located in the interior of the shell and wound around a portion of the core adjacent to but outside of the working air gap for altering the magnetic field across the working air gap in response to changes in electric current passing through the auxiliary coil to increase the maximum force which may be developed by the drive coil.

11. Apparatus according to claim 10 in which the air outlet is located in the cylinder so that downward vertical movement of the piston in the cylinder forces air out through the air outlet.

12. Apparatus according to claim 10 in which the guide means comprises a cooperating shoulder and groove arrangement between the drive coil and the core for producing guided vertical movement of the drive coil along the core.

13. Apparatus according to claim 10 in which the core has a first portion of its length disposed in the working air gap and a second-portion of its length located adjacent to but outside of the working air gap, the drive coil is located in the working air gap and surrounds the first portion of the core, the drive coil also being connected to a first source of electrical power and being movable back and forth relative to the first portion of the core in the magnetic field in response to changes in electrical current passing through it, the

auxiliary coil is wound around the second portion of the core, the drive coil is movable toward and away from the auxiliary coil, and the auxiliary coil is connected to a second source of electrical power independent of the power source associated with the drive coil for varying the magnetic field across the working air gap.

14. Apparatus according to claim 13 in which the auxiliary coil is independent of the drive coil and is mounted in a stationary position in the shell interior.

15. Apparatus according to claim in which the auxiliary coil is located at the end of the shell interior adjacent the respirator cylinder.

16. An air delivery system for a respirator which ineludes a closed cylinder having a hollow interior, a movable piston in the interior of the cylinder for forcing air through an air outlet in the cylinder, and a linear drive for moving the piston, the linear drive comprising a closed shell adjcent the cylinder, the shell having an outer wall with a pair of closed ends enclosing a hollow interior therein, an elongated core extending through the hollow interior of the shell from one closed end to the other, permanent magnet means in the interior of the shell surrounding a predetermined length of the core, the permanent magnet means being spaced laterally from the core to form a narrow working air gap between the permanent magnet means and the core, with a magnetic field being established across the working air gap, the core having a first portion of its length disposed in the working air gap and a second portion of its length located adjacent to but outside ofthe working air gap, a movable drive coil located in the working air gap and surrounding the first portion of the core, the drive coil being connected to a first source of electrical power and being movable back and forth relative to the first portion of the core in the magnetic field in response to changes in electric current passing through it, drive means extending outwardly from the drive coil through corresponding openings in the shell outer wall for attachment to the piston for applying force to the piston in response to movement of the drive coil, and an auxiliary coil located within the interior of the shell and wound around the second portion of the core such that the drive coil is movable toward and away from the auxiliary coil, the auxiliary coil being connected to a second source of electrical power independent of the power source associated with the drive coil for varying the magnetic field across the working air gap in response to changes in electric current passing through the auxiliary coil to increase the maximum force which may be applied by the drive coil to the respirator piston.

17. Apparatus according to claim 16 in which the core comprises a solid steel body having an outer sur' face, the working air gap is a void which surrounds the core and extends from the outer surface of the steel core to the permanent magnet means, and the width of the working air gap is just sufficient to allow passage'of the drive coil between the outer surface of the core and the permanent magnet means.

18. Apparatus according to claim 17 including a cooperating shoulder and groove arrangement between the drive coil and the core for producing guided movement of the drive coil along the core.

19. Apparatus according to claim 16 in which the auxiliary coil is independent of the drive coil and is mounted in a stationary position in the shell interior.

20. Apparatus according to claim 16 in which the piston, in normal use, is movable vertically back and forth in the interior of the cylinder, and in which the core extends vertically through the hollow interior of the shell, the drive coil being supported on the core to slide vertically back and forth relative thereto to apply vertical translational motion to the piston.

21. Apparatus according to claim 20 in which the air outlet is located in the cylinder so that downward vertical movement of the piston in the cylinder forces air out through the air outlet.

22. Apparatus according to claim 20 in which the auxiliary coil is located at the end of the shell interior adjacent the respirator cylinder.

23. In an air delivery system for a respirator which includes a cylinder and a piston movable in the cylinder for forcing air to a patient through an air outlet in the cylinder, a linear drive for moving the piston, the linear drive comprising a. a shell adjacent the cylinder and having an outer wall with a pair of closed ends enclosing a hollow interior of the shell,

b. an elongated core extending through the hollow interior of the shell between the closed ends thereof, the core consisting essentially of a ferromagnetic material having an outer surface thereof spaced from the interior of the shell outer wall,

0. permanent magnet means in the hollow interior surrounding a predetermined length of the core, the permanent magnet means being spaced from the core to form a narrow working air gap comprising a void which surrounds the core and extends from the outer surface of the core to the permanent magnet means, whereby a magnetic field is established across a narrow working air gap,

d. a movable drive coil'surrounding the portion of the core located in the working air gap, the width of the working air gap being just sufficient to allow passage of the drive coil between the outer surface of the core and permanent magnet means, the drive coil being movable along the length of the core in the magnetic field in response to changes in electric current passing through the drive coil,

e. drive means extending outwardly from the drive coil through corresponding elongated openings in the shell outer wall and connecting the piston to the drive coil to apply a reciprocating translational motion to the piston in response to movement of the drive coil, and

f. an auxiliary coil located in the interior of the shell and wound around the portion of the core adjacent to but outside of the working air gap for altering the magnetic field across the working air gap in response to changes in electric current passing through the auxiliary coil to increase the maximum force which may be developed by the drive coil. 

1. A permanent magnet translational motor comprising a shell having an outer wall with a pair of closed ends enclosing a hollow interior, an elongated core extending through the hollow interior of the shell from one closed end to the other, permanent magnet means in the interior of the shell surrounding a predetermined length of the core, the permanent magnet means being spaced laterally from the core to form a narrow working air gap between the permanent magnet means and the core, with a magnetic field being established across the working air gap, the core having a first portion of its length located in the working air gap and a second portion of its length located adjacent to but outside of the working air gap, a movable drive coil located in the working air gap and surrounding the first portion of the core, the drive coil being connected To a first source of electrical power and being movable back and forth relative to the first portion of the core in the magnetic field in response to changes in electric current passing through it, the drive coil having opposed mounting means extending outwardly therefrom through corresponding openings in the shell outer wall for attachment to a load for applying force to the load in response to movement of the drive coil, and an auxiliary coil located within the interior of the shell and wound around the second portion of the core such that the drive coil is movable toward and away from the auxiliary coil, the auxiliary coil being connected to a second source of electrical power independent of the power source associated with the drive coil for varying the magnetic field across the working air gap in response to changes in electric current passing through the auxiliary coil to increase the maximum force which may be developed by the drive coil.
 2. Apparatus according to claim 1 in which the magnetic field established by the permanent magnet means defines a closed path extending in opposite directions along the core, across the closed ends of the shell, and along the outer wall of the shell.
 3. Apparatus according to claim 1 in which the shell is cylindrical, and the core is a cylindrical body extending coaxially along the longitudinal axis of the cylindrical shell, the drive coil being an annular body adapted to move back and forth in an enclosed annular working air gap formed between the permanent magnet means and the core.
 4. Apparatus according to claim 1 in which the auxiliary coil is independent of the drive coil and held in a stationary position in the shell interior.
 5. Apparatus according to claim 1 in which the core is a solid steel body having an outer surface, the working air gap is a void which surrounds the core and extends from the outer surface of the core to the permanent magnet means, and the width of the working air gap is just sufficient to allow passage of the drive coil between the outer surface of the core and the permanent magnet means.
 6. Apparatus according to claim 5 including a cooperating shoulder and groove arrangement between the drive coil and core for producing guided movement of the drive coil along the core.
 7. A permanent magnet translational motor comprising a closed steel shell having an outer wall with a pair of closed ends enclosing a hollow interior, a core comprising an elongated solid steel body extending continuously through the hollow interior of the shell from one closed end to the other, the core having an outer surface spaced from the interior of the shell outer wall, permanent magnet means located in the interior of the shell and surrounding a predetermined length of the core, the permanent magnet means being spaced from the core to form a narrow working air gap comprising a void which surrounds the core and extends from the outer surface of the core to the permanent magnet means, whereby a magnetic field is generated across the narrow working air gap, the core having a first portion of its length disposed in the working air gap and the second portion of its length adjacent to but outside of the working air gap, a movable drive coil located in the working air gap and surrounding the first portion of the core, the width of the working air gap being just sufficient to allow passage of the drive coil between the outer surface of the core and the permanent magnet means, the drive coil being connected to a first source of electrical power so it will move back and forth relative to the first portion of the core in the magnetic field in response to changes in electric current passing through it, opposed drive means extending outwardly from the drive coil through corresponding openings in the shell outer wall for attachment to a load to supply force to the load in response to movement of the drive coil, and a stationary auxiliary coil located in the interior of the shell anD wound around the second portion of the core, the auxiliary coil being adjacent to but outside of the working air gap so that the drive coil is movable toward and away from the stationary auxiliary coil, the auxiliary coil being independent of the drive coil and being connected to a second source of electrical power independent of the power source associated with the drive coil for varying the magnetic field across the working air gap in response to changes in electric current passing through the auxiliary coil to increase the maximum force which may be developed by the drive coil.
 8. Apparatus according to claim 7 in which the magnetic field established by the permanent magnet means defines a closed path extending in opposite directions along the core, across the closed ends of the shell, and along the outer wall of the shell.
 9. Apparatus according to claim 7 in which the drive means extend radially outwardly through corresponding longitudinally extending slotted openings in the outer wall of the shell; and including a cooperating shoulder and groove arrangement between the core and the drive coil for producing guided movement of the drive coil along the core.
 10. An air delivery system for a respirator which includes a closed cylinder having a hollow interior, a piston which in normal use is movable vertically up and down in the interior of the cylinder for forcing air to a patient through an air outlet in the cylinder, and a linear drive for moving the piston, the linear drive comprising a closed steel shell adjacent the cylinder, the shell having an outer wall with a pair of closed ends enclosing a hollow interior of the shell, an elongated core extending vertically through the hollow interior of the shell from one closed end to the other, the core being a solid steel body having an outer surface spaced from the interior of the shell outer wall, permanent magnet means in the hollow interior surrounding a predetermined length of the core, the permanent magnet means being spaced from the core to form a narrow working air gap comprising a void which surrounds the core and extends from the outer surface of the core to the permanent magnet means, whereby a magnetic field is established across the narrow working air gap, a movable drive coil surrounding the portion of the core located in the working air gap, the width of the working air gap being just sufficient to allow passage of the drive coil between the outer surface of the core and the permanent magnet means, guide means for slidably engaging the drive coil with the core to allow the coil to move up and down vertically on the core and in the magnetic field in response to changes in electric current passing through the drive coil, drive means extending outwardly from the drive coil through corresponding elongated openings in the shell outer wall and connecting the piston to the drive coil to apply reciprocating translational motion to the piston in response to vertical movement of the drive coil, and an auxiliary coil located in the interior of the shell and wound around a portion of the core adjacent to but outside of the working air gap for altering the magnetic field across the working air gap in response to changes in electric current passing through the auxiliary coil to increase the maximum force which may be developed by the drive coil.
 11. Apparatus according to claim 10 in which the air outlet is located in the cylinder so that downward vertical movement of the piston in the cylinder forces air out through the air outlet.
 12. Apparatus according to claim 10 in which the guide means comprises a cooperating shoulder and groove arrangement between the drive coil and the core for producing guided vertical movement of the drive coil along the core.
 13. Apparatus according to claim 10 in which the core has a first portion of its length disposed in the working air gap and a second portion of its length located adjacent to but outside of the working air gap, the drive coil is located in the woRking air gap and surrounds the first portion of the core, the drive coil also being connected to a first source of electrical power and being movable back and forth relative to the first portion of the core in the magnetic field in response to changes in electrical current passing through it, the auxiliary coil is wound around the second portion of the core, the drive coil is movable toward and away from the auxiliary coil, and the auxiliary coil is connected to a second source of electrical power independent of the power source associated with the drive coil for varying the magnetic field across the working air gap.
 14. Apparatus according to claim 13 in which the auxiliary coil is independent of the drive coil and is mounted in a stationary position in the shell interior.
 15. Apparatus according to claim 10 in which the auxiliary coil is located at the end of the shell interior adjacent the respirator cylinder.
 16. An air delivery system for a respirator which includes a closed cylinder having a hollow interior, a movable piston in the interior of the cylinder for forcing air through an air outlet in the cylinder, and a linear drive for moving the piston, the linear drive comprising a closed shell adjcent the cylinder, the shell having an outer wall with a pair of closed ends enclosing a hollow interior therein, an elongated core extending through the hollow interior of the shell from one closed end to the other, permanent magnet means in the interior of the shell surrounding a predetermined length of the core, the permanent magnet means being spaced laterally from the core to form a narrow working air gap between the permanent magnet means and the core, with a magnetic field being established across the working air gap, the core having a first portion of its length disposed in the working air gap and a second portion of its length located adjacent to but outside of the working air gap, a movable drive coil located in the working air gap and surrounding the first portion of the core, the drive coil being connected to a first source of electrical power and being movable back and forth relative to the first portion of the core in the magnetic field in response to changes in electric current passing through it, drive means extending outwardly from the drive coil through corresponding openings in the shell outer wall for attachment to the piston for applying force to the piston in response to movement of the drive coil, and an auxiliary coil located within the interior of the shell and wound around the second portion of the core such that the drive coil is movable toward and away from the auxiliary coil, the auxiliary coil being connected to a second source of electrical power independent of the power source associated with the drive coil for varying the magnetic field across the working air gap in response to changes in electric current passing through the auxiliary coil to increase the maximum force which may be applied by the drive coil to the respirator piston.
 17. Apparatus according to claim 16 in which the core comprises a solid steel body having an outer surface, the working air gap is a void which surrounds the core and extends from the outer surface of the steel core to the permanent magnet means, and the width of the working air gap is just sufficient to allow passage of the drive coil between the outer surface of the core and the permanent magnet means.
 18. Apparatus according to claim 17 including a cooperating shoulder and groove arrangement between the drive coil and the core for producing guided movement of the drive coil along the core.
 19. Apparatus according to claim 16 in which the auxiliary coil is independent of the drive coil and is mounted in a stationary position in the shell interior.
 20. Apparatus according to claim 16 in which the piston, in normal use, is movable vertically back and forth in the interior of the cylinder, and in which the core extends vertically through the hollow interior of the shell, the drive coil being supPorted on the core to slide vertically back and forth relative thereto to apply vertical translational motion to the piston.
 21. Apparatus according to claim 20 in which the air outlet is located in the cylinder so that downward vertical movement of the piston in the cylinder forces air out through the air outlet.
 22. Apparatus according to claim 20 in which the auxiliary coil is located at the end of the shell interior adjacent the respirator cylinder.
 23. In an air delivery system for a respirator which includes a cylinder and a piston movable in the cylinder for forcing air to a patient through an air outlet in the cylinder, a linear drive for moving the piston, the linear drive comprising a. a shell adjacent the cylinder and having an outer wall with a pair of closed ends enclosing a hollow interior of the shell, b. an elongated core extending through the hollow interior of the shell between the closed ends thereof, the core consisting essentially of a ferromagnetic material having an outer surface thereof spaced from the interior of the shell outer wall, c. permanent magnet means in the hollow interior surrounding a predetermined length of the core, the permanent magnet means being spaced from the core to form a narrow working air gap comprising a void which surrounds the core and extends from the outer surface of the core to the permanent magnet means, whereby a magnetic field is established across a narrow working air gap, d. a movable drive coil surrounding the portion of the core located in the working air gap, the width of the working air gap being just sufficient to allow passage of the drive coil between the outer surface of the core and permanent magnet means, the drive coil being movable along the length of the core in the magnetic field in response to changes in electric current passing through the drive coil, e. drive means extending outwardly from the drive coil through corresponding elongated openings in the shell outer wall and connecting the piston to the drive coil to apply a reciprocating translational motion to the piston in response to movement of the drive coil, and f. an auxiliary coil located in the interior of the shell and wound around the portion of the core adjacent to but outside of the working air gap for altering the magnetic field across the working air gap in response to changes in electric current passing through the auxiliary coil to increase the maximum force which may be developed by the drive coil. 